When designating the “best system of emission reduction” in its Clean Power Plan, EPA considered several factors far beyond the fencelines and control of the regulated power plants. The clear statutory language, context, and regulatory background demonstrate that such a “beyond-the-source” approach is not allowed under §111. To find otherwise would suggest that EPA can require drivers to stay home or to use public transportation in order to reduce motor vehicles’ tailpipe emissions under the Clean Air Act. Although this conclusion may result in lower overall emission reductions, it is the outcome that the CAA requires.